United Airlines has again been sued by a passenger for mistreatment, after he was pushed to the floor by a United employee in 2015 at George Bush Intercontinental Airport in Houston. The airline issued an apology, but only after video of the episode was made public.

The Houston television station KPRC broke the story on Tuesday when it obtained the surveillance video, from July 21, 2015, showing a customer service agent shoving a 71-year-old man to the floor near check-in kiosks. The video shows the man lying motionless and unattended despite the presence of several United employees in the area. After nearly a minute, another passenger approaches and tells the employees to call 911.

The release of the video follows a high-profile altercation in April, when a United passenger, David Dao, was dragged violently from his seat on an overbooked flight after he refused to give it up.

“I don’t think this is United specifically,” said Gary Leff, the author of the travel blog Viewfromthewing.com. “But U.S. airlines generally have not been good at screening for the best talent and screening out people who are not good at customer service.”

United issued a statement to KPRC describing the employee’s behavior as “completely unacceptable” and noting that he is no longer with the company.

The passenger, Ronald Tigner of Houston, has filed a lawsuit against the airline, claiming negligence, for more than $1 million.

The episode began when Mr. Tigner requested a new boarding pass, saying that the original was illegible, but was denied by the airline, KPRC reported. Security agents with the Transportation Security Administration rejected the ticket, and when Mr. Tigner returned to the counter he was again rebuffed. According to the report, the United employee, Alejandro Anastasia, said, “Can’t you see we’re busy?” Mr. Tigner then told him to “wipe that smile off your face,” after which Mr. Anastasia swore at him and pushed him to the ground, where Mr. Tigner remained motionless for several minutes.

The dispute is the latest in a series of recent conflicts between airlines and passengers. In April, a Delta Air Lines employee threatened a California family with jail time over a seat dispute, and an American Airlines flight attendant was caught on video challenging a passenger to a fight after he stood up for another intimidated passenger.

Many of these encounters have been caught on cellphone video, prompting the airlines to apologize and in some cases to make financial amends. “Otherwise it’s treated as something that didn’t happen or it’s someone’s word against someone else’s,” Mr. Leff said. “If you don’t have that viral media potential, you don’t have impact.”